1. Field of the Invention
This invention relates to forming pressed paperboard products such as paper trays and plates and to the apparatus for forming such products.
2. Description of the Prior Art
Formed fiber containers, such as paper plates and trays, are commonly produced either by molding fibers from a pulp slurry into the desired form of the container or by pressing a paperboard blank between forming dies into the desired shape. The molded pulp articles, after drying, are fairly strong and rigid but generally have a rough surface and are not usually coated so that they are susceptible to penetration by water, oil and other liquids. Pressed paperboard containers, on the other hand, can be decorated and coated with a liquid resistant coating before being stamped by the forming dies into the desired shape. Pressed paperboard containers generally cost less and require less storage space than the molded pulp articles. Large numbers of paper plates and similar products are produced by each of these methods every year at relatively low unit cost. These products come in many different shapes, rectangular or polygonal as well as round, in multicompartment configurations.
Pressed paperboard containers tend to have somewhat less strength and rigidity than do comparable containers made by the pulp molding processes. Much of the strength and resistance to bending of a plate-like container made by either process lies in the side wall and rim areas which surround the center or bottom portion of the container. When in use, such containers are supported by the rim and side wall while the weight held by the container is located on the bottom portion. Thus, the rim and side wall generally is placed in tension when the container is being used.
In plate-like structures made by the pulp molding process, the side wall and overturned rim of the plate are unitary, cohesive fibrous structures which have good resistance to bending as long as they are not damaged or split. Because the rim and side wall of the pulp molded containers are of a cohesive, unitary structure, they may be placed under considerable tension without failing.
In contrast, when a container is made by pressing a paperboard blank, the flat blank must be distorted and changed in dimension in order to form the generally planar blank into the desired three dimensional shape. This necessary distortion results in seams or pleats in the sidewall and rim, the areas of the container which are reduced in press forming the container. These seams or pleats constitute material fault lines in the side wall and rim areas about which such containers bend more readily than do containers having unflawed side walls and rims. Moreover, such seams or pleats have a tendency to return to their original flat shape. The necessary location of these pleats in the side wall and rim of pressed paperboard containers places the greatest weakness in the area requiring the greatest strength. Such containers have been unable to support loads comparable to pulp molded containers since, when in use, the greater the load the higher the tension imposed on the rim and sidewall. Imposing tension on pleats merely enhances the tendency to flatten. Accordingly, known pressed paperboard containers have significantly less load carrying ability than do pulp molded containers. A pressed paperboard plate being less costly than its pulp molded competitor would have significant commercial value if it had comparable strength and ridigity.
Many efforts have been made to strengthen pressed paperboard containers while accommodating the necessary reduction in area at the side walls and rims. Blanks from which paperboard containers are pressed have been provided with score lines at their periphery to eliminate the random creation of seams or pleats. The score lines define the locations of the seams or pleats. Score lines, sometimes in conjunction with special die shapes, have been used to create flutes or corrugations in the sidewall and rim for aesthetic and structural purposes. The additional cost and complexity of dies used to create flutes or corrugations in the side wall of such containers is a cost disadvantage, and the containers are not significantly more rigid than prior paperboard containers.
Whether the area reduction of the side wall and rim is accommodated by pleats, seams, flutes or corrugations, the basic difficulty has been that under limited stress the paperboard will tend to return to its original shape.
To overcome this tendency, it has been suggested that the rim be subjected to various strengthening techniques. The earliest efforts comprised the addition of several thicknesses of paperboard at the rim. This container, however, required additional manufacturing steps and increased the cost and required storage space of the containers. Examples of this technique are disclosed in Moore, U.S. Pat. No. 2,627,051, and Bothe, U.S. Pat. No. 2,668,101.
Wilson, British Pat. No. 981,667, teaches subjecting the lip or rim of the container to pressure greater than that imposed on the rest of the container in the belief that the additional compression would resist the tendency of the rim to return to its original shape. While the rim of the device of Wilson is flattened, the side wall of the container is corrugated presenting the disadvantages referred to above.
More recently, as disclosed in a commonly-assigned, copending U.S. application, Ser. No. 764,965, filed Aug. 12, 1985, improved rigidity in a pressed paperboard container has been achieved by application of pressure and temperature to the rim of the container while applying substantially no pressure to the sidewall and bottom wall. In particular, the container had a generally planar bottom wall, a side wall upwardly rising from the bottom wall periphery and an overturned rim extending from the sidewall periphery. During integrally press-forming of the container, substantially no pressure was applied to the bottom and side walls and pressure was applied to the overturned rim. The amount of pressure imposed on the rim was approximately 200-250 psi and gradually increased from the juncture of the rim and side wall to the periphery edge of the rim. The pleats formed in the rim were compressed to the thickness of the rim while the pleats formed in the side wall were not subject to any significant pressure. The container thus formed provided a significant improvement over prior paperboard containers.
Another commonly-assigned, co-pending U.S. application, Ser. No. 697,888, filed Feb. 4, 1985, discloses a pressed paperboard container of improved strength and a method of making it. The paperboard container is comprised of a bottom wall, an upwardly extending side wall, a first curved portion joining the side wall to the periphery of the bottom wall, an outwardly extending rim, a second curved portion joining the rim to the periphery of the side wall, and a downwardly curved lip outwardly extending from the periphery of the rim. The container is integrally formed from a substantially homogeneous paperboard blank by a press such that the thickness of the side wall, second curved portion and rim is less than that of the bottom wall, first curved portion and lip. The container includes a plurality of densified regions radially extending through and circumferentially spaced about annular sections of the side wall, second curved portion and rim. The densified regions are formed from pleats including at least three layers of paperboard created during press forming of the blank which are subjected to sufficient pressure to reform the pleats into cohesive, fibrous structures having a density substantially greater than and a thickness substantially equal to adjacent areas of the side wall, second curved portion and rim. The application also discloses a method of forming containers from a flat, substantially homogeneous paperboard blank comprising shaping the blank into a formed container having a bottom wall, an upturned side wall extending from the bottom wall, a rim outwardly extending from the side wall and a lip downwardly extending from the rim and including pleats formed in the side wall, rim and lip. Sufficient heat an pressure are applied to the side wall and rim to decrease their cohesive fibrous structures having a density greater than and a thickness substantially equal to adjacent areas of the side wall and rim.
It is a principal object of the present invention to provide a method and an apparatus for forming a container from a blank of sheet material when the outer peripheral portion of the container is formed after the interior portion of the container, thereby reducing stress on the blank during its formation into the container. An additional object of the invention is to provide an apparatus for economically forming a container from a blank of sheet material. An additional object of the invention is to form a container from a blank of sheet material having a downturned lip portion that is unwrinkled. A further object of the invention is to form a container from a blank of sheet material without applying excessive pressure to the lip portion during formation.
Additional objects of the invention will be apparent from the following description of the preferred embodiments or can be learned by practice of the invention.